The video shows a militant with a British accent beheading Goto. Called "A Message to the Government of Japan," it includes the symbol of ISIS' al-Furqan media arm, and is also similar to previous beheading videos released by the extremists. The video was released on militant websites, and highlighted by militant sympathizers on social media sites. It has not been authenticated, but world leaders have already condemned the killing.

On Sunday, Japanese Prime Minister Shinzo Abe spoke to reporters about the video, and expressed his condolences to Goto's family.

"The government has tried its best to deal with this murder, but we are deeply saddened by this despicable and horrendous act of terrorism," he said. "We denounce it on the strongest terms. To the terrorists: We will never, never forgive them for this act."

Abe said Japan will not give in to "terrorism," and added that he would continue Japan's humanitarian aid to the Middle East.

U.S. President Barack Obama also denounced Goto's beheading in a statement.

"The United States condemns the heinous murder of Japanese citizen and journalist Kenji Goto by the terrorist group ISIL. Through his reporting, Mr. Goto courageously sought to convey the plight of the Syrian people to the outside world," Obama said, adding that U.S. stands in solidarity with Japan. "Standing together with a broad coalition of allies and partners, the United States will continue taking decisive action to degrade and ultimately destroy ISIL."

UK Prime Minister David Cameron went as far as to call the militants the "embodiment of evil" on Twitter.

I condemn what appears to be the despicable and appalling murder of Kenji Goto. It is a reminder that ISIL is the embodiment of evil.

Throughout the approximately minute-long video, Goto, who is kneeling in an orange prison jumpsuit, does not say anything.

In a message directed to Abe, the militant says in the video, "Because of your reckless decision to take part in an unwinnable war, this man will not only slaughter Kenji, but will also carry on, and cause carnage wherever your people are found. So let the nightmare for Japan begin."

Goto's mother, Junko Ishido, told NHK TV she wants her son to be remembers for his kindness and courage. His brother, Junichi Goto, told the Japanese broadcaster, according to BBC: “I was hoping Kenji would come back alive to thank everyone who had supported him. I am filled with sadness he couldn’t do it.”

The SITE Intelligence Group, a U.S.-based terrorism monitor, said on Saturday that ISIS released a video of the beheading.

Rita Katz, the director of SITE, described comments purportedly made in the video by a militant named "Jihad John." The ISIS militant, also known as "Jihadi John," murdered journalists James Foley and Steven Sotloff, as well as other high-profile prisoners. He is the same person who has appeared in previous videos released by the militant group.

Katz said the video did not mention the fate of another ISIS prisoner, Lt. Moaz al-Kassasbeh, a Jordanian pilot.

ISIS had called for a prisoner swap, and said it would release Goto, 47, in exchange for Sajida al-Rishawi, 46, a failed suicide bomber who has been held in Jordan since 2005. On Tuesday, Goto purportedly appeared in a recording that claimed he and Kassasbeh, 26, would be killed in 24 hours if Jordan did not free Rishawi.

Sky Sources: Uncle of Jordanian pilot currently being held hostage by Islamic State says the family is "devastated by death" of #KenjiGoto

Japan's deputy foreign minister, Yasuhide Nakayama, said late Friday that efforts to free Goto were "in a state of deadlock." Jordan and Japan reportedly conducted indirect negotiations with the militants through Iraqi tribal leaders.

Last Saturday, a video message purportedly from ISIS claimed that Haruna Yukawa, 42, a second Japanese hostage who was held with Goto, had been killed. Goto was captured in October, after he traveled to Syria to try to win Yukawa's release.

The hostage crisis began on Jan. 20, after an online video purportedly showed ISIS threatening to kill Goto and Yukawa in 72 hours unless it received a $200 million ransom.

There was growing pressure on the Japanese government to free Goto, spurred by the online hashtag campaign #IamKenji, according to the Committee to Protect Journalists.

Goto founded a news agency, called Independent Press, in 1996. The father of three reported on conflict in the Balkans, Africa and the Middle East, and as a freelancer, he funded many of his own reporting trips.

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